1. Technical Field
The present invention relates to a method of transmitting bursts in a digital broadcasting network, particularly, but not exclusively, to a method of transmitting bursts in a digital video broadcasting (DVB) network.
The present invention also relates to a network element for preparing bursts to be transmitted in a digital broadcasting network, particularly, but not exclusively, to a multiprotocol encapsulator (MPE) for use in a DVB network.
The present invention further relates to a terminal for receiving data from a digital broadcasting network, particularly, but not exclusively to a mobile, handheld terminal for receiving data from a DVB network
2. Discussion of Related Art
ETSI EN 302 304 “Digital Video Broadcasting (DVB); Transmission System for Handheld Terminals (DVB-H)” V1.1.1 (2004-06) specifies a system for delivering multimedia services via DVB networks to mobile, handheld terminals. The system is based on DVB data broadcasting specified in ETSI EN 301 192 “Digital Video Broadcasting (DVB); DVB Specification for data broadcasting” V1.4.1 (2004-06).
In DVB data broadcasting, data is delivered to a terminal in datagrams, which are placed in sections using Multi-Protocol Encapsulation (MPE). MPE sections in turn are mapped into MPEG-2 Transport Stream (TS) packets in accordance with ISO/IEC Standard 13818-1 “Information Technology-Generic coding of moving pictures and associated audio information: Systems” for transmission.
It is more difficult to receive data at a mobile, battery-powered terminal, than it is to a static, mains-powered terminal. For example, mobile reception is particularly prone to impulse noise. Furthermore, terminal battery power is limited.
DVB-H employs forward error correction (FEC) to provide robustness against noise and uses time slicing to allow a terminal to conserve battery life.
Data and associated FEC parity data may be prepared in a data frame known as an MPE-FEC frame and reference is made to Clause 9.3 in EN 302 304 ibid.
A conventional MPE-FEC frame is arranged as a matrix with 255 columns and a flexible number of rows. The number of rows is signaled in service information (SI) and can have a value of up to 1024. Each position in the matrix holds a byte of data and so an MPE-FEC frame can hold up to almost 2 Mbits of data.
The left part of the MPE-FEC frame, consisting of the 191 leftmost columns, is dedicated to OSI layer 3 (in other words, Network layer) datagrams, such as Internet Protocol (IP) datagrams, and possible padding, and is called the application data table. The right part of the MPE-FEC frame, consisting of the 64 rightmost columns, is dedicated for the parity information of the FEC code and is called the RS data table. Each byte position in the application data table has an address ranging from 0 to 191×no_of_rows−1. Similarly, each byte position in the RS data table has an address ranging from 0 to 64×no_of_rows−1.
Layer 3 datagrams are introduced datagram-by-datagram, starting with the first byte of the first datagram in the upper left corner of the matrix and going downwards to the first column. The length of the datagrams may vary arbitrarily from datagram to datagram. Immediately after the end of one datagram, the following datagram starts. If a datagram does not end precisely at the end of a column, it continues at the top of the following column. When all datagrams have entered the application data table any unfilled byte positions are padded with zero bytes, which makes the leftmost 191 columns completely filled.
With all the leftmost 191 columns filled it is now possible, for each row, to calculate the 64 parity bytes from the 191 bytes of data and possible padding. The code used is Reed-Solomon RS (255,191,64). Each row then contains one RS codeword. Some of the rightmost columns of the RS data table may be discarded and hence not transmitted, to enable puncturing. The exact number of punctured RS columns does not need to be explicitly signaled and may change dynamically between frames. Thus, the RS data table is filled and the MPE-FEC frame is completed.
IP datagrams are mapped into MPE sections and RS columns are mapped into MPE-FEC sections.
The datagrams are carried in MPE sections in compliance with the DVB standard, irrespective of whether MPE-FEC is being used. This makes reception fully backwards compatible with MPE-FEC ignorant receivers. Each section carries a start address for the payload in the section header. This address indicates the byte position in the application data table of the first byte of the section payload. In case the datagram is divided over multiple MPE sections, each MPE section indicates the byte position in the application data table of the first byte of the datagram fragment carried within the section. The receiver will then be able to put the received datagram in the right byte positions in the application data table and mark these positions as “reliable” for the RS decoder, provided the section CRC-32 check shows that the section is correct.
The last section of the application data table contains a table boundary flag, which indicates the end of the datagrams within the application data table. If all previous sections within the application data table have been received correctly the receiver does not need to receive any MPE-FEC sections and, if time slicing is used, the receiver can be switched off without receiving and decoding RS data. If MPE-FEC sections are also received, the number of padding columns (columns filled with padding bytes only) in the application data table is indicated with 8 bits in the section header of the MPE-FEC sections; this value is used if RS decoding is performed.
RS columns are carried in MPE-FEC sections. Each section carries exactly one column of the RS data table. Punctured columns are not transmitted and not signalled explicitly.
Data can be transmitted in time-slicing bursts and reference is made to Clause 9.2 in EN 302 304 ibid.
Time slicing involves sending data in bursts using significantly higher bit-rate compared to the bit-rate required if the data was transmitted using conventional bandwidth management. Within a burst, the time before the start of the next burst (delta-t) is indicated. Between the bursts, data of the elementary stream is not transmitted, allowing other elementary streams to use the bandwidth otherwise allocated. This enables a receiver to stay active only for a fragment of the time, while receiving bursts of a requested service.
Conventionally, the contents of one MPE-FEC frame is transmitted in one time-sliced burst. However, this arrangement suffers at least two drawbacks.
Firstly, the interleaving length of the application data and RS data is short and so the burst is still prone to noise. One solution is to lengthen the duration of the burst. However, this reduces the benefit of time slicing.
Secondly, if a burst carries data for more than one service, then the same level of error protection is used.